1. Field of the Invention
The present invention relates to screen process printing plates for use in fabrication of thick film integrated circuits, printed circuit boards, hybrid circuits, etc. and particularly it relates to a screen process printing plate which enables printing with high accuracy and durability.
2. Description of the Related Art
Screen process printing is carried out according to the following procedures. A screen process printing plate is prepared in a manner in which figures or patterns consisting of openings and non-open portions are formed mainly by a photomechanical process on a screen stretched on a screen frame. Printing liquid such as printing paste or printing ink is put on the screen process printing plate. An instrument called a squeegee is used to slide over the surface of the screen while being pressed against the surface. The ink is squeezed out from the openings and the figures or patterns are transferred onto a surface of an object subjected to printing placed under the screen.
Referring to FIGS. 1 and 2, the screen frame 1 includes a rectangular frame body 2 of wood or metal, and a screen 3 attached to the frame body 2. The four sides of the screen 3 are fixed to the frame body 2 with a prescribed tension by using adhesives or other suitable methods.
Referring to FIG. 3, when printing is to be carried out, an object 5 subjected to printing is placed on a printing table 4 by vacuum contact or other method. The frame body 2 is fixed to allow a gap G between the material 5 and the screen 3 and the screen frame 1 is set in the main body of a printing apparatus. The screen 3 is stretched horizontally as shown by the broken line. Ink 6 is applied to the screen 3. The screen 3 is pressed by the squeegee 7 and bent, so that the screen 3 contacts the surface of the material 5. The screen 3 is stretched to the position shown by the solid lines. While the squeegee 7 moves in the direction shown by the arrow P, the ink 6 is squeezed out and transferred onto the material 5 through the openings of the screen 3.
As the squeegee 7 advances, the position of contact between the screen 3 and the material 5 moves. The screen 3 adheres to the material 5 at the contact position due to viscosity of the ink. However, when the squeegee 7 further moves, the screen 3 at the previous contact position gets away from the material 5 due to its own tension. This phenomenon is called "snapping-off".
The above-mentioned screen generally includes woven meshes of textile structure consisting of fibers of silk, nylon, polyester, or the like. In some cases, a woven, metallic mesh of stainless steel may be used. In other cases, the screen includes a metallic mesh formed by an etching method applied to metal such as stainless steel.
Another example of a construction of a conventional screen process printing plate is disclosed, for example, in Japanese Utility Model Publication No. 55-1894. This screen process printing plate is of a so-called "combination system". Referring to FIGS. 4 to 6, the screen process printing plate 8 of the combination system includes a frame 2, an elastic film 9 attached at four sides thereof to the frame 2 with a prescribed tension, having a rectangular opening at its center, and a mesh type screen 10 of metal or the like connected to the elastic film 9 in overlap portions 11. The above-mentioned printing pattern is formed on the screen 10.
Referring to FIGS. 5 and 6, ink 6 is put on the screen 10. The squeegee 7 is operated to slide over the screen 10 while being pressed against it. Thus, the ink 7 is squeezed out through the printing pattern to the lower surface of the screen 10, whereby the prescribed pattern is transferred onto the object 5 subjected to printing.
Referring again to FIG. 3, the screen 3 stretches elastically in the printing process. As a result, the printing pattern formed on the screen 3 is deformed.
Similarly, the screen 10 of the combination system as shown in FIG. 6 also stretches elastically in the printing process. However, in the screen process printing plate 8 of the combination system, the elastic stretching of the screen 10 in the printing process is mainly borne by the elastic film 9. Stretching of the portion of the screen 10 where the printing pattern is formed is suppressed. Accordingly, there is little deformation of the printing pattern. In this regard, the screen process printing plate 8 of the combination system is excellent compared with the screen process printing plate 1 shown in FIGS. 1 to 3.
However, in either case, the screen process printing plate 1 as shown in FIGS. 1 to 3 or the screen process printing plate 8 of the combination system as shown in FIGS. 4 to 6 includes the rectangular frame body having its four sides fixed, and the screen stretched on the frame body 2. At the time of screen process printing, the squeegee slides over the upper surface of the printing plate while pressing against it. The screen is elastically stretched and deformed. Depending on the rigidity of the frame body 2, deformation in the frame body would occur in an extreme case.
In addition, the influence exerted by the frame body 2 on the deformation of the screen differs depending on the position of the squeegee. More specifically, distribution of force applied to the respective sides of the frame body 2 and to the screen differs depending on whether the squeegee 7 is located at the center of the screen process printing plate or near an end of the printing pattern. Consequently, when the squeegee 7 moves near an end of the printing pattern, abnormal deformation of the printing pattern might occur.
The recent trend in screen process printing technology is toward higher precision and higher resolution. Generally, the number of meshes per unit length of the screen is increased, in order to meet the above-mentioned demand. Thus, it is necessary for the screen to have fine meshes.
In consequence, problems as described below occur. Consider, for example, a screen of a metallic mesh formed by a plating method. It is believed that the metallic screen is not elongated very much by the pressing of the squeegee. However, distortion per unit load increases with increasing fineness of the mesh. As a result, the mesh is liable to be elastically deformed. If a relatively large tension is continuously applied to the screen, permanent deformation of the screen by a creep phenomenon is also liable to occur due to the fineness of the mesh. As a result, stretch deformation or distortion of the printing pattern occurs.
One method for solving the above-described problems, involves fixing only the opposite two sides of the screen. One side is fixed to a fixed frame body and the opposite side is fixed to a movable frame body provided in parallel with the fixed frame body. Further details of the method are below.
Referring to FIG. 7, a screen frame 110 includes a fixed frame body 112 fixed to the main body of a printing apparatus (not shown), a screen 116 having one end fixed to the fixed frame body 112, a movable frame body 114 attached to the other end of the screen 116, and a tension spring 60 biasing the movable frame body 114 toward the side opposite to the fixed frame body 112 to stretch the screen 116. A printing plate portion 118 is formed in the central portion of the screen 116.
In order to avoid unevenness of distribution of tension, it is necessary for the screen 116 to be reinforced. The screen 116 is reinforced for example by strip portions 120 formed on opposite two sides of the screen 116 along the printing direction of the squeegee 122 with the printing plate portion 118 being provided therebetween.
The existence of the strip portions 120 for the reinforcement brings about the below-described effect. When the screen 116 is pulled by the movable frame body 114, most of the tension is applied to the strip portions 120. There is little tension applied to the screen 116. Thus, the screen 116 is unlikely to be deformed by tension. Accordingly, creep distortion hardly occurs in the screen 116.
Various reinforcing methods may be considered. The first method is as follows. In a manufacturing process of a screen process printing plate, a photosensitive material is applied to the whole surface of a screen and a figure or a pattern is formed in a central portion of the screen by a photomechanical process. During the process, two side edges of the screen are reinforced by the photosensitive material remaining on the surface of the screen. The second method is to attach metallic belts to a screen. The third method is to form metallic strips on the surface of a screen by plating.
However, in the screen process printing by using the above-described screen frame 110, the following disadvantages may occur. Referring to FIG. 8, the width W0 of the squeegee 122 is smaller than the width W1 of the screen 116 (corresponding to a dimension between the strips 120 for reinforcing in the screen frame 110 shown in FIGS. 7 and 8) (W0&lt;W1). The center of the squeegee 122 in the widthwise direction substantially coincides with the center of the screen 116 in the widthwise direction at the time of printing. The central portion of the screen 116 is pulled down by the squeegee 122 and is stretched downward.
The reinforcing strips 120 and the other portions including the printing plate portion 118 have different stretching amounts in the squeegee running direction when the same tension is applied thereto. Excess force is applied to the screen 116 at the junction between the reinforcing strips 120 and the other portions including the printing plate portion 118. The screen 116 is bent in those portions, causing disadvantages such as breakage of the screen 116. Particularly, if the strips 120 are formed by plating metallic strips on the screen 116, or if the strips 120 are formed simultaneously with preparing the screen by plating, such disadvantages would be liable to occur.